netdev - Re: [PATCH 4/4] phylib: Allow reading and writing a mii bus from atomic context.

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Message-ID: <20100716112544.GA2996@riccoc20.at.omicron.at>
Date:	Fri, 16 Jul 2010 13:25:47 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Andy Fleming <afleming@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 4/4] phylib: Allow reading and writing a mii bus from
 atomic context.

On Wed, Jul 07, 2010 at 09:18:32AM +0200, Richard Cochran wrote:
> Consider the receive path:
> 
> 1. PTP Packet passed through PHY. PHY recognizes it and stores a time
>    stamp along with some UID from the packet.
> 
> 2. Napi calls the MAC driver's poll function.
> 
> 3. MAC driver acquires packet. At this point, if we want to have a
>    hardware time stamp, we must read it out over the mdio bus, before
>    handing the packet over to the stack via netif_receive_skb().
> 
> If we decide to defer the packet delivery (in step 3), we have to know
> whether the PHY will have a time stamp for this packet. The only way
> to do this is to compare the UIDs, but that requires reading it over
> the mdio bus.

(Forwarding Andy's message to me that was missing a CC to the list)

On Thu, Jul 08, 2010 at 03:40:41PM -0500, Andy Fleming wrote:

Wait, is the intent for this mdio read to be done for *every* packet?
MDIO is spec'ed out to go up to 2.5MHz.  Each transaction takes 64
cycles.  And I see that reading the timestamp from the PHY you
submitted support for takes at *least* 2 transactions.  The fastest
you can process packets would then be under 20,000 packets per second.
Even on a 100Mb link with full-sized packets, you would be 40% done
receiving the next packet before you had passed the packet on to the
stack.  Each MDIO transaction takes 25,600 cycles on a gigahertz
processor.  It's just too long, IMO, and it looks like this code will
end up doing up to...10?

I wasn't able to find an example of how you were going to use the
time-stamp functions you provided.  Could you please go into a little
more detail about how you intended this to work?
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